If $500 was my max i would go a used 10", telescopes don't out date like computers lol, well apart from mirror damage over time. Lol you always end up with a few dings in newts and this helps shave some price off used, even though there going to function the same. Your also got to think about moving it about, a bigger scope may turn you off star gazing, i sure as hell hate packing the 10" in the car though for around the home it rulez.
GSO offers the best bang for buck scopes, i find skywatcher a little better with construction and there mirrors use pyrex glass which cool quicker then the bk7 glass. Though really for viewing on a dob with a fan added there not really alot of difference. Stay away from Bushnell and other cheaper brands, theres really only GSO and skywatcher and then there oem's aka bintel (GSO re-badged and saxon skywatcher re-badged). There are a few other brands Orion and celestron as well meade and even a lot more, though these scopes usually come at a higher cost and there are not many Australian dealers. The rule of thumb is that you pay for what you get in nearly all cases, apart from being raped by the australian re-sellers lol.
Anyhow you can't go wrong with a newt, they just can't be beaten for bang for buck. There's a few issues like coma and collimation, but these can be dealt with, don't buy a cheap laser collimator as it will rob you of to much joy.
Also thanks to notebooks these days put some settings circles on that dob, grab your self a copy of stellarium (100% free) here
http://www.stellarium.org/ and your got a self power goto scope, even son powered haha.